GEOMORPHOLOGY

CRATER GEOMORPHOLOGY

1. INTRODUCTION

GEOMORPHOLOGY
The scientific study of the origin and evolution of topographic and bathymetric features created by physical, chemical or biological processes operating at or near the Earth’s surface. The following examples illustrate the shortcoming of using geomorphology in crater identification:

Alsever Lake (image LEFT) is located at the southern boundary of Algonquin Park. It is similar in appearance to the Brent impact crater (image RIGHT) at the northern boundary of Algonquin Park. Both structures have two distinct bodies of water forming a circular patterns.

Scootamatta Lake (image LEFT) is located in Southern Ontario. It is similar in appearance to the Presqu’ile impact crater (image RIGHT) located in north central Quebec. Both structures have distinct bodies of water forming circular patterns.

Fault zone and differing geology under the Charlevoix impact structure. I superimposed a schematic of the geologic faulting under the Charlevoix crater onto this aeronautical chart of the crater area. A line drawn west to east on the chart corresponds to the physical position of the schematic illustrating the fault zone and differing geology under the crater.Charlevoix impact structure Digital Elevation Model with Earthquake Epicentres

Structural contour map on the pennsylvanian Tensleep Formation (modified from Stone 1999). Contour interval is 200 ft (60 m). Base level datum is mean sea level. The trace of the outer rim fault zone just below the Tr-J unconformity is marked by the largest dashed circle. The smallest dashed circle near the center of the map is the annular fault zone that encircles the central peak. Within this annular peak-ring fault zone (the area marked by a gray stippled pattern), the coutoured tensleep horizon is missing (by ejection and/or erosion) and Amsden Formation rocks are uplifted and truncated at the Tr-J unconformity.Structural cross-section B-B’ restored by flattening on the Sundance datum to remove the effects of Laramide deformation. Formation symbols are identified in the figure to the left. (Stone et al 2003). Comparison of the “standard” stratigraphic column on the Casper Arch and the section found in the central peak of the Cloud creek impact structure. Stratigraphic uplift (SU) of the Madison Formation in the central peak area above its normal elevation outside the crater is ~520 m. At the Tr-J unconformity, there is progressive onlap of Sundance members over and around the central peak (Stone et al 2003).

Topography and bathymetry of Deep Bay. Also indicated are location of drill holes and a geologic cross-section based on these drill holes (bottom). Innes et al (1964). PDF in quartz grains and feldspar were recovered from the drill site DOM 66-1 (Dence et al 1968..At Deep Bay the circular fracture zone marks approximately the outer limit of the fractured zone and may be interpreted as a hinge line about which the granitic rocks forming the rim have been uplifted. Indeed the rocks within this circle, have the general appearance of having been uplifted and shattered into huge blocks without having undergone much horizontal movement. (Innes et al 1964).

The 8-km diameter Des Plaines Structure exhibits complex faulting and shock features such as percussion fractures and strain lamellae, as well as a few shatter cones. The center of the crater lies under Big Bend Lake on the Des Plaines River. Seismic reflection data suggest that there are numerous other faults within the bedrock of Cook County. Courtesy of United States Meteorite Impact Craters.

The (WISCAH), evaluation presents evidence that the black mat stratum at Glover Bluff is the depositional result of an ET event due to its clear association with ejecta strata. Additional geological analysis is recommended at Glover Bluff to establish the exact timing, and to determine the relationship, if any, to known phenomena. The impact origin was confirmed in 1983, by the discovery of shatter cones (Read, 1983).

Profile of the Holleford Crater as reconstructed from drill-hole and surface observations. It will be seen that the original crater surface dips nearly 800 feet below plain level, while the zone of fractured rock extends to an estimated depth of about 2,400 feet. The estimate of breccia depth at the centre depends on theoretical considerations advanced by J.A. Rothenberg.Looking North from the South East rim, note the “steep scarps”.

3-D perspective of the Maple Creek (White Valley) structure. Both the Belly River (RED) and Mississippian (BLUE) horizons are depicted. The general morphology of the complex crater can be seen in the Belly River horizon. (Westbroek et al 1995).The trough and central uplift are imaged by seismic data (Westbroek, 1997).The Maple Creek (White Valley) structure in southwestern Saskatchewan has been interpreted as a <75 Ma old impact feature. This structure has many of the morphological characteristics of a complex impact crater. The structure is interpreted to have a diameter of about 7 km with an annular trough and a raised central uplift.

Whitecourt Crater – perspective view from SE. This image is derived by Light Detection And Ranging (LiDAR) technology. (Department of Earth and Atmospheric Sciences, University of Alberta)Cross sections of the ejecta blanket along 038° and 110° with a reference figure showing the location of the sections. Approximate distribution of the ejecta blanket and the main soil pit and auger hole site locations are also provided. (Kofman et al – Meteoritics & Planetary Science 2010)

3. SUSPECTED IMPACT

Ground penetrating radar survey and interpretation of the Bloody Creek structure. Ground-penetrating radar traverses were obtained when the site was frozen in winter. They confirm the crater morphology of the structure, and the distinction of the shallow levels from the undisturbed bedrock. (Spooner et al 2009)

Seismic data in the James River 3-D volume. Several interpreted horizons are shown. The upper horizon corresponds to the top of the Cambrian, the middle horizon corresponds to the Cambrian ‘event’, and the lower horizon corresponds to the Precambrian. Faulting in the James River dataset is divided between shallow rim faults and deep central and rim faults.

The layers of sedimentary rock that covered the crater sagged, stretching the rock at the rim causing differencial cracking. The fractures (joints) developed above the rims of the crater were eroded during the last ice age. The glacial erosion of the joints which formed about the crater rim formed the course of the Esopus and Woodland Creeks.

The Victoria Island structure is characterized by a concentric, annular, terraced rim and trough surrounding a structurally uplifted central peak. Contour lines of equal thickness over an area (Isopach map) of interval between upper Nortonville Shale marker and lower Domengine Formation marker, showing series of concentric circular ridges and troughs, together with positions of several major, curvilinear normal faults that surround the structure and cut the lower part of the isopached interval.Contour lines of equal thickness over an area (Isopach map) of potential crater infill, between upper Nortonville Shale marker and base Nortonville Shale/top Domengine Formation marker.